This invention relates to improvements in apparatus for eliminating or reducing backlash and/or freeplay between a pair of meshing gears, and in particular between a worm gear and a worm wheel in an electric power assisted steering system.
It is known in the art to provide an electric power assisted steering system of the kind in which an electric motor applies an assistance torque to a steering column in response to a measure of the torque applied to the steering column by the driver of a vehicle. The motor acts upon the steering column through a worm gear and wheel gear with the worm gear being provided on an output shaft of the motor and the wheel being provided on a driven shaft which may comprise a portion of the steering column. The worm and wheel act as a reduction gearbox between the motor and the driven shaft. Such a system will hereinafter be referred to as being xe2x80x9cof the kind set forthxe2x80x9d.
EP-A-420131 shows an electric power steering system having the features of the pre-characterising portion of claim 1.
EP-A-270159 shows a system for automatically taking up play in a worm gear, the system comprising a compression spring.
A problem with known steering systems of the kind set forth, particularly where the driven shaft comprises a section of the steering column shaft, is that any free play (backlash) between the teeth of the worm and the teeth of the wheel can result in intermittent knocking noises upon torque load reversal. These noises can arise both as a result of variations in the level or torque applied by the motor and also due to movement of the steering column shaft due to the actions of the driver and vibration in the steering rack due to imperfections in the road surface or poorly balanced road wheels.
The problem of noise and knocking due to backlash, has, in practice been found to be significant even at quite low levels of backlash. For example, backlash can cause a problem even at levels as low as 0.015 mm, whereas the lowest tolerance range for backlash that can typically be achieved during production assembly (without individual piece-by-piece adjustment) is around 0.50 mm. In addition, it is known that backlash may increase during use of the system due to wear, by approximately 0.05 mm. This is noticeable when plastic gears are used to reduce friction weight and cost. Metallic gears would provide an increased resistance to wear but would not be able to provide the low friction levels and tolerance of marginal lubrication needed to ensure that the unit remains back drivable over its working life.
An object of the present invention is therefore to provide a means of overcoming, at least partially, the problem of backlash within the gears between the worm gear and the wheel gear.
According to our invention in an electric power steering system of the kind set forth, the motor output shaft which incorporates a worm gear is supported within a housing by at least two support assemblies spaced axially along the output shaft, a first one of said support assemblies including means to allow radial displacement of an end of the output shaft relative to the wheel gear whilst the other support assembly is adapted to allow angular displacement of the output shaft relative to the housing, characterised in that the first support assembly comprises a bearing assembly supported in an opening or recess provided eccentrically within a bush.
The provision of the support assemblies is advantageous because by allowing radial displacement of at least one end of the output shaft, the spacing between the worm gear and worm wheel can be varied. This allows the free play and backlash in the gearset to be adjusted.
The wheel gear may have a fully-throated or half throated tooth form in which the throat radius is substantially greater than (i.e. xe2x80x9cnon-conformalxe2x80x9d with) the radius of the worm. Thus, a relatively large degree of variation in the positioning, axially, of the wheel gear relative to the worm gear can be tolerated without producing large changes in backlash and free playxe2x80x94whilst only a small change in the radial displacement between the worm gear and the wheel gear produces a useful change in backlash levels.
An advantage of the non conformal throat form is that it is easier to machine than a conformal throat form because it allows a larger diameter of hob and a more advantageous shape for the cutting teeth.
Preferably, the first one of said support assembles support the output shaft substantially at its free end, i.e. the end furthest from the electric motor. The other support assembly preferably supports the output shaft at a point in between the motor assembly and the worm gear, perhaps approximately midway along the output shaft.
Preferably, the motor output shaft, worm gear and the worm wheel on the steering column shaft are all provided at least partially within a single common housing.
The bush may be located in an opening or recess in the housing. Because the bearing is mounted eccentrically in the bush, rotation of the bush about its centre of rotation causes radial displacement of the output shaft relative to the housing at least where it passes through or enters the bush.
Where the output shaft is supported in at least two support assemblies and the first one is adjusted to provide radial displacement of the shaft, the angle at which the output shaft will pass through the other support assembly will vary. Accordingly, the other support assembly is adapted to allow a degree of angular displacement.
The first support assembly may be manually adjusted to facilitate the radial displacement of the output shaft. For example, the bush may incorporate a flange portion which is secured to the housing. The flange portion may be secured by one or more bolts or screws which pass through elongate openings in the flange. The openings allow the flange (and hence the eccentric bush) to be rotated manually relative to the bolts when the bolts arc loosened. Once the correct position is achieved the bolts (or screws or the like) may be tightened to fix the flange securely in place.
In an alternative arrangement, the first support means may be adapted to automatically adjust during use to displace the output shaft radially. This could, for instance, allow the shaft to be moved automatically to maintain the free play and/or backlash at a desired level.
The automatic adjustment means may comprise a compliant member which is adapted to apply a load between the output shaft and the worm wheel through the worm gear. The load may be of a fixed value or may be adjustable. The compliant member may comprise a tension spring or a compression spring. The spring may be linear or non-linear, ie a clock spring.
The compliant member may be adapted to exert a force between the housing and the eccentric bush which tends to rotate the bush. The rotation is resisted by the force of the worm gear acting upon the worm wheel. The force exerted by the compliant member is preferably chosen so that friction between the parts of the support means and the housing is overcome.
The second support assembly may comprise a first plate and a second plate which clamp a bearing race therebetween. Each plate may include one or more raised protrusions which clamp the center race of the bearing assembly. Most preferably, each plate has a central bore through which the output shaft can pass and a pair of diametrically spaced protrusions either side of the bore facing the bearing assembly when clamped. The bearing assembly is preferably clamped over substantially its control third portion only by the protrusions so that it can xe2x80x9ctiltxe2x80x9d or pivot about the protrusions. Thus, a degree of angular rotation of the bearing races is possible which allows small tolerance bearings to be used.
The protrusions may be hardened pads, and the bearing races may have a high axial stiffness to prevent excessive uneven loading of the ball bearings in the horizontal plane when axial forces act on the bearing.
An advantage of the automatic adjustment means is that it eliminates substantially the need to manually adjust the position of the output shaft during use, allowing wear in the gearset to be automatically corrected.
In addition the automatic adjustment means, a manual backup may also be included to enable fine adjustment of the output shaft position to be achieved.